The use of multiple transmit/receive antennas in wireless networks promises mitigation of interference and high spectral efficiencies through concentrating signals along a designated direction or transmission path. Compared to single-antenna-to-single-antenna transmissions, transmit beamforming may yield increased range (e.g., an N-fold increase for free space propagation), increased rate (e.g., an N2-fold increase in a power-limited regime), increased power efficiency (e.g., an N-fold decrease in the net transmitted power for a fixed received power), and/or may allow splitting a high data-rate stream into multiple lower data-rate streams. (Here, N is the number of cooperative nodes or antenna elements at the transmit side.)
Distributed coherent RF transmit beamforming is a form of cooperative communication in which two or more information sources simultaneously transmit a common message, controlling the phase and delay of their transmissions so that the signals constructively combine at an intended destination. The term “beamforming” may also be used to indicate what is more commonly referred to as directional beamforming. In this case the information sources are configured to produce a beam that is approximately collimated in a given direction and the beam is not specifically focused to maximize power at any one location, but only in one direction. Phased arrays where the locations of the individual elements and the target receiver are known, where the array elements are interconnected with cables or other calibrated interconnections, and where a common centralized clock/time reference can be distributed among the array elements, can be configured to operate in such directional beamforming mode.
However, decentralized arrays, where the nodes are independent untethered devices with independent clocks i.e., without distributed clock or frequency reference, and where the positional coordinates are unknown, are much more difficult to use as coherent phased arrays, either in transmit mode or receive mode. For such systems of devices to operate as phased arrays, they should perform two major tasks.
First, as with any phased array, they must acquire the correct channel information between the array members and the intended target/source and provide a mechanism for the nodes to transmit/receive a correctly weighted signal at each of the array elements so that beamforming is achieved to within an accuracy required by the system.
Second, the array should implement a distributed algorithm across the members of the array that enables the array to operate in a coherent manner, providing phase, frequency, and time alignment of the clocks and oscillators of the different array members of the array. A correct method of producing this coordination of the array members is essential to the correct operation of the phased array.
Since multiple clocks are used across the array, the algorithm should operate fast enough to provide the required alignment within time limits determined by the clock coherence.
Even with atomic clocks, the clock coherence limit is eventually reached. In a phased array, exceeding the coherence limit may manifest as a random scrambling of the phases of the carrier waves utilized in the beamforming and hence a failure to achieve optimal or even minimally acceptable performance.
To correct for this, the algorithm should be compatible with the requirement that the system alignment be periodically refreshed to compensate for limited clock coherence and for operation of the array in dynamic and changing channels.
Another desirable characteristic is that the algorithm be capable of aligning the system (array members/elements/nodes) in a manner that minimizes the required information sharing and other communication between the array members.
In what follows, we will describe the arrays in transmit mode, for simplicity. But the ideas are easily applied to receive mode operations.
In general, existing implementations fall into three broad categories. First there are those categories where the array transmitters are required to determine or calculate the correct beamforming weights to focus at a known target and where the location of the transmitters and target is known. This is the most common type of phased array implementation and is often referred to as an “open-loop” array. Such implementation clearly requires time, frequency, and phase synchronization across the array. Because the various nodes may send signals at different times with different phase offsets, they should all agree on a common time base and a method for ensuring that the signals arrive at the target with their carriers at the same frequency and in perfect phase alignment to avoid fading. FIG. 7 illustrates selected elements of Open Loop Basic Array, wherein N untethered, randomly distributed array elements are configured to operate as a basic phased array to produce beamforming to a target location determined by the array itself. The formula in the Figure, s(t)=Σi=1Nαi(t−(τi+T))cos(ω(t−(τi+T))), means that the signal at the target is a summation of the signals produced at the target by the elements i of the array, each with a time-dependent envelope αi, delayed by the corresponding time of flight (τi+T). The array does not require communication (information flow as such) from the target location. It simply calculates the required delay offsets to ensure that the signals from each element are aligned at the target (some designated beamfocus point). The location of the target may be defined with respect to a reference point on the array, or elsewhere. When the locations of the array elements are fully known and the clocks of the elements are synchronized, it is relatively simple to operate such an array. If the clocks are not synchronized, and/or if the element locations are unknown, however, such arrays are not well suited for beamfocusing, because it is difficult or impossible to calculate how far the elements are from the focus point (the target), nor can it be accurately specified when each element should launch its signal. This array configuration would typically be used for LoS targets, since, by definition, the array has no information regarding the channel impulse response other than the simple straight line distance to the target. To focus in NLoS environments generally requires detailed knowledge of the channel impulse response of the NLoS channel. If this channel information could be made available for each node element through some other means (e.g., information flow from the target to the individual array elements), then it would be possible to focus to a NLoS environment. But this is not defined as part of the array configuration properties.
A second general category of array architectures is the “retrodirective array,” described with reference to FIG. 8 and FIG. 9. The formula in FIG. 8 is the same one as in FIG. 7 (s(t)=Σi=1Nαi(t−(τi+T))cos(ω(t−(τi+T)))), and has the same meaning. In this array configuration, the array is assumed to be untethered and randomly distributed. The basic principle of the retrodirective array is that the signal acquisition phase (i.e., sounding process) enables the array to measure and calculate a set of relative delays τi of the signal coming from the target and arriving at each node. Conventional State of the art arrays attempt to measure the delays of the arriving signals and to send a signal in return where each array element is adjusted in time to compensate for this delay so that all the signals return to the target node at the same time. Proper design may enable a retrodirective array to operate with the full array gain. Retrodirective arrays generally can operate in NLoS channels (as well as in LoS channels), since the array elements have the information about the NLoS channel from the sounding process and channel reciprocity. The knowledge of the X, Y, Z coordinates of the array elements is not required for proper operation of the main channel between the cooperative array and the target. Accurate clock synchronization, however, is needed.
This type of array is not strictly open loop, since it requires a sounding pulse or an opportunistic signal detected from the target. It is also not a closed loop system in the conventional sense, since there is no transmission of information or messages back and forth between the array and the target for control and alignment purposes (see concept of cooperative arrays below). In a closed-loop system, the target receiver determines what alignment is required by the transmitter nodes and sends that information back to the transmitters. In the system described above, the transmitter array determines the correct procedure and need not receive feedback from the target receiver informing it how well aligned the system (the array) is.
In principle, all control of the operation of the array may be performed at the array end, and the sounding pulse is simply a way to acquire channel information. This type of array may be very important in dynamic array configurations in noisy environments. Because the array operation does not require information transmission across the channel for alignment purposes, it may be less susceptible to jamming and interference. The process of clock synchronization of independent and untethered clocks, however, does require transmission of information back and forth between the array elements, and may present a challenge and be susceptible to interference.
In a third category of array architectures, the target receiver is capable of communicating with each transmit array node, and the target can determine when optimal beamforming has been achieved. In such array configurations, the array is assumed to be untethered and randomly distributed (ad hoc). The target is assumed to be a cooperative node and be capable of sending information to the array. The array operation can be controlled, for example, from the target or from the array itself, but the assumption is that a full closed loop operation is used that enables effective alignment, when both ends of the channel are capable of communicating information relevant to the beamfocusing operation. It is usually assumed that the target has the ability to determine when optimal beamforming has been achieved. For example the target may be able to assess the power density of the focused spot, or it may be able to measure BER in a communication system. It may also be required to determine from this signal how to adjust the required parameters of the array to optimize or improve the beamforming and to be able to communicate appropriate signals to the array to achieve this goal. It is the responsibility of the target to return control signals to the transmitters instructing them to modify their beamforming weights until the target determines that optimal beamforming has been achieved. Systems like these are often referred to as “closed-loop” systems. In this approach, neither the transmitters nor the receiver may have perfect channel state information, but there is a low-rate feedback link from the receiver to the transmitters. In various applications this arrangement may not require time or phase synchronization across the members of the array because the receive node is assumed to be capable of instructing the transmitters to adjust their parameters to achieve an optimal alignment.
Time synchronization across the array elements might make the performance of such arrays much faster and simpler, but it is not required, because the target may attempt to determine the clock properties of each array element and send its delay correction information in a manner that takes into account the clock properties of each array member. The target may be responsible for handling the synchronization of the array, since it is uniquely positioned to determine the array performance and how the performance responds to changes at the array end of the channel.
This type of array configuration typically demands a large alignment overhead and may have problems rapidly adapting to motion or changes in the channel properties. It may also be susceptible to jamming and interference, since it requires a low error rate communication channel to be available between opposite ends of the channel between the target and the array.
Irrespective of the particular category described above, the resultant beam shape at the receiver may resemble a phased-array radiation pattern, with one main lobe and multiple undesired side lobes that cause interference. In conventional phased-array systems, it may also be difficult or impossible to support coherent addition of wave-fronts in multipath (MP) environments, and most beamforming approaches assume line-of-sight (LoS) links between transmitters and receiver.
The problem of array alignment becomes rather difficult when the individual member of the transmitter array are free to move with respect to each other, and do not share a common local oscillator (LO) reference, because the phases/frequencies may vary from one array member to another, and because the timing of transmission may change as the elements move with respect to each other and with respect to the receiver, as is typical in dynamic environments. The movements and changes in the channel may seriously degrade the alignment required for reliable collaborative communications in an ad hoc array system.
Needs exist in the art for improved communication techniques for distributed coherent communications, and for apparatus, methods, and articles of manufacture enabling such improved communications. Needs exist in the art for phase/frequency synchronization techniques that can be used in ad hoc nodes of a distributed transmitter array for coherent transmissions.